Recently, in keeping with studies of technologies to effect optical transmission on a circuit board whereby an electrical signal inputted and outputted at a semiconductor device is converted into an optical signal before transmission, active researches and development have been made on optical lines including an optical waveguide as a transmission line for the signals.
The use of an optical line enables not only the speeding up of signal transmission but also reduction in crosstalk noise between signals and achievement of ever-finer and ever-denser electrical lines.
In general, a semiconductor device is mounted on a surface of a package board made of a ceramic or plastic material. Hereafter a package board will be also referred to as “a semiconductor package”.
A semiconductor package is mounted on and electrically connected to a circuit board. The circuit board has electrical lines adapted to input signals and output signals to and from the semiconductor package. The electrical lines allow input and output of electrical signals between the semiconductor package and the circuit board. Moreover, on a surface layer of the circuit board is formed an electrode pad facing to a metal pad of the semiconductor package. The electrode pad and the metal pad are connected together by a solder ball.
In an optical communication device disclosed in Japanese Unexamined Patent Publication JP-A 2003-215372, a semiconductor photo diode and a semiconductor light emitting device are mounted on a semiconductor package. The semiconductor package has a through optical transmission body for establishing optical connection between the top surface and the back surface of the semiconductor package. Meanwhile, a circuit board of this construction is formed with an optical waveguide, an end face of which is shaped into a 45 degree-angled turning mirror. A solder resist layer is formed on a surface layer of the optical waveguide. An aperture for optical path is formed around the turning mirror.
The semiconductor package and the circuit board are connected to each other by a solder ball for mutual electrical conduction.
Positioning of the semiconductor package and the circuit board is effected by exploiting a self-alignment effect exhibited by the solder ball, for example.
In this case, however, there arises a distance between the semiconductor package and the circuit board by the dimension of the solder ball, which results in optical leakage between the semiconductor package and the circuit board. This leads to the problem of occurrence of inter-channel cross talk and loss of optical transmission.
Furthermore, positioning and mounting based on the self-alignment effect are techniques that depend heavily on the proficiency in operation of a skilled and trained worker. It is therefore quite difficult to adopt such a technique as a mounting method for electronic equipment that is required to be mass-produced at low cost.